DOI: 10.1021/la1007473 11657Langmuir 2010, 26(14), 11657–11662 Published on Web 06/21/2010

pubs.acs.org/Langmuir

© 2010 American Chemical Society

Synthesis of TiO2 Nanocrystals with a High Affinity for

Amine Organic Compounds

Ricardo H. Gonc-alves,*,† Wido Herwig Schreiner,‡ and Edson R. Leite*,†

†Department of Chemistry, Federal University of S~ao Carlos, 13565-905 S~ao Carlos, SP, Brazil, and‡Departament of Physics, Federal University of Paran�a, 81531-990, Curitiba PR, Brazil

Received February 20, 2010. Revised Manuscript Received April 14, 2010

This article describes a different approach to the colloidal synthesis of TiO2 nanocrystals using a polymer melt as asolvent. This approach allowed us to obtain a colloidal dispersion with a high degree of stability in a polymeric solvent,resulting in a transparent colloid. Using this method, it was possible to obtain the TiO2 nanocrystal with Brønsted acidsites and polymer chains chemically anchored on the nanocrystal surface. The acid surface of those nanocrystals has thechemical property to react in the presence of amine organic compounds and to maintain the colloidal stability. In thisway, TiO2 nanocrystals were combined with a molecular probe containing amine functional groups such as polyaniline.Through the combination of the molecular probe and inorganic nanocrystals, we obtained a hybrid material withinteresting chemical, optical, and electronic behavior, making it a promising material for photovoltaic, photochromic,and sensor devices.

Introduction

Titaniumoxide nanocrystals have attracted considerable atten-tion because of their importance in a wide variety of applicationsincluding photocatalysts, biosensors, andmainly in solar cells.1-4

The synthesis of colloidal TiO2 nanocrystals with controlled sizeand shape is the key step in these technological applications. Thenonaqueous synthesis route of TiO2 nanocrystals is a sophisti-cated method for producing nanoparticle with a narrow sizedistribution and high crystallinity.5-11 Nevertheless, this methoddoes not ensure the formation of colloids. The most importantfactor in colloid formation is the stability of the system, whichdepends on particle size and mainly the surface chemical compo-sition. In particular, the control of a colloidal surface is not asimple task, and many studies have revealed the achievement ofthe colloidal stability of TiO2 nanocrystals via chemical surfacemodification using a specific ligand, such as carboxylic acid,

organosilanes, or phosphorus organometallics.12-16 Organic mo-lecules with carboxylic acid groups have been the principalcompound used to modify to the surface of inorganic nanocryst-als because this compound acts as a chelating agent.17 In manysituations, the carboxylic acid is only a functional group anchor-ing the specific part of the molecule.18 However, there is a hugevariety of amine compounds with interesting biological andchemical properties that could be directly anchored to the TiO2

surface, resulting in a material with different functionality. Forinstance, some ruthenium organometallics or bioligands19,20

could be chemically incorporated onto the TiO2 surface withoutthe addition of a carboxylic acid functional group to the molec-ular structure. However, the amine group shows a weak affinityfor the surface of TiO2 obtained in aqueous media or via thesynthesis method that uses surfactants as ligands.21,22

The inherent acid-base property of the solid surface is a criticalparameter in evaluating the interactions with chemical com-pounds.23Miyamoto et al. have shown that the terminal hydroxylgroups on the surface of vanadiumpentoxide act as Brønsted acidsites for ammonia adsorption and that these acid sites are moreenergetically stable.24 Typically, the chemical strategy for increas-ing the affinity between the inorganic surface and amine com-pounds is to increase the degree of dissociation of the Brønstedacid sites on the solid surface. In addition, the number of the acidsites depends on the surface area, crystallographic planes, andresidual ligands.25-27 In general, the synthesis approach used in

*Corresponding authors. E-mail:ricardohg.ufscar@gmail.com and derl@power.ufscar.br.(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38.(2) Paunesku, T.; Rajh, T.; Wiederrecht, G.; Maser, J.; Vogt, S.; Stoji�cevi�c, N.;

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the nanocrystals synthesis is the key factor in controlling theacidic nature of the surface. It is well accepted that some surfac-tants and solvents inactive the acid sites during the synthesisprocess, drastically reducing the density of acid sites and theaffinity with specific compounds in a posterior synthesis step.28-30

In this article, we introduce a different concept of the TiO2

surface, using appropriate synthesis methods that allow us toobtain an oxide undergoing a strong interaction with an amineorganic compound without additional postprocessing steps. Wesynthesized anatase TiO2 nanocrystals with Brønsted acid sites;consequently, this surface has a strong affinity for basic organiccompounds such as organic molecules with amine functionalgroups. TheTiO2 nanocrystalswere prepared in a polymermelt asa solvent, and they play an important role in the reaction process,such as reagent and solvent.31 To identify the Brønsted acid siteson the TiO2 nanocrystal surface, we select a molecular probe thatpresents a strong interaction with this kind of acid site andpresents different properties after the protonation reaction. Thepolyaniline is a conducting polymer that is well known for its easysynthesis, environmental stability, and unique acid/base doping/undoping and oxidation/reduction chemistry. In the emeraldineoxidation state (emeraldine base), polyaniline becomes electri-cally conducting when doped with acid, leading to the formationof emeraldine salt.32 The tunable color and conductivity can beused to detect Brønsted acid sites on the solid surface. Manyreports showed the combination of TiO2 and doped polyanilineusing a simple mixing process.33-36 However, it is important topoint out that we are using the undoped polyaniline as amolecular probe to characterize the strong acid sites on thesurfaces of TiO2 nanocrystals and to obtain a hybrid materialwith polymer directly bound to the inorganic surface. Further-more, these hybrid materials are an alternate path to obtainingnew functionality as a result of the combination between organicand inorganic properties.37,38

Experimental Section

Materials. Titanium(IV) chloride (TiCl4, 99.99%, Aldrich),poly(1,4-butanediol) (HO(CH2CH2CH2CH2-O)n-H), averagemolecular weight Mw = 1000 g/mol, Aldrich), emeraldine base(Mw ≈ 5000 g/mol, Aldrich), N,N-dimethylformamide (DMF),and acetone were used.

Synthesis of TiO2 Nanocrystals. TiCl4 (10 mmol) wasinjected into 20 g of poly(1,4-butanediol) at 80 �C in a nitrogenatmosphere. This transparent solutionwas heated to120 �Cundervigorous stirring and then kept for 6 h in a closed glass flask. Theresulting TiO2 nanocrystals were cooled to 60 �C, after which60mLofacetonewas added.This solutionwas then centrifuged at1400 rpm for 5 min; after that it was washed several times with

acetone. Part of the sample was vacuum dried at 60 �C for 6 h forlater characterization. The other part of the sample was used toprepare a standard colloidal TiO2 solution by adding 10 mL ofDMF to obtain a TiO2 concentration of 37 mg/mL.

Molecular Probe Test. Eight milligrams of emeraldine basewas dissolved in 2 mL of DMF, forming a blue solution. Thissolution was then mixed with 2 mL of standard colloidal TiO2

solution, which instantly changed color to green. This greensolution was centrifuged at 1400 rpm for 15min and washed withDMF three times to remove excess polyaniline, after which 50%of the sample was vacuum dried at 70 �C for 18 h, yielding into agreen powder that was subjected to characterization. The otherhalf of the sample was dispersed again in DMF for characteriza-tion by UV-vis spectroscopy.

Characterization

Transmission Electron Microscopy. The size and morpho-logy of the TiO2 nanocrystals were determined by high-resolutiontransmission electron microscopy (HRTEM), using a TECNAIF20 FEI microscope operating at 200 kV. The TiO2 nanocrystalswere dispersed in ethyl alcohol, which was applied to an ultrathin400 mesh carbon-coated grid.Powder X-ray Diffraction (XRD). The crystallographic

phase of theTiO2 nanocrystal powderwas determined on aRigakuD/MAX-2500diffractometerusingCuKR radiation (λ=1.5418 A).The scanning step size was 0.02� in 2θ with a counting time of 1 sper step.Dynamic Light Scattering (DLS). DLS analysis was per-

formed in a 90 plus particle size analyzer (Brookhaven Instru-ments) at a laser wavelength of 660.0 nm and a detection angle of90� at 20 �C. The TiO2 was dispersed in DMF solvent, and theDLS analysis was adjusted to the viscosity and refractive index ofthis solvent.Infrared Spectroscopy. Fourier transform infrared spectros-

copy (FT-IR) analyses were obtained from a solid sample of TiO2

mixed with KBr powder using a Bruker Equinox 70 FT-IRapparatus in reflectance mode at a resolution of 4 cm-1 over a4000-500 cm-1 spectral range.Thermogravimetric Analysis (TGA). TGA was performed

on a Netzsch model STA 409 to determine organic mass lossunder an oxygen flux at a rate of 10 �C min-1.UV-Vis SpectroscopicAnalysis.UV-vis spectroscopywas

essential to garnering information about the absorbance behaviorof TiO2-doped polyaniline nanocrystal.UV-vis absorption spectrawere obtained using a Shimadzu 3100 UV-vis spectrophoto-meter.Photoactivity. The photoactivity test was carried out with a

30 W UV lamp as the excitation source, exposing the samples to30 min of ultraviolet radiation. A similar experiment was realizedwith a 150 W visible-light lamp.X-ray Photoelectron Spectroscopy (XPS). The XPS mea-

surements were taken in a VG Microtech ESCA3000 spectro-meter with a base vacuum of 3 � 10-10 mbar, a 250 mmsemihemispherical analyzer with nine channeltrons, and Al KRnonmonochromatic radiation. Binding energy corrections weremade to the raw spectra using the handbook reference of Ti 2p3/2and the C 1s peak at 284.5 eV for the N 1s peak.Conductivity Measurement. Four-probe pressed pellet con-

ductivities for the TiO2-emeraldine salt sample were carried outusing a Keithley 237 high-voltage source measurement unit atroom temperature. The current-voltage curve of the pressedpellet exhibited nonlinear behavior. The conductivity measure-ment was taken after the postrupture voltage in the ohmic region.Photoluminescence (PL). PL spectra were collected with a

Jobin-Yvon, Inc. FluorologmodelFL3-12.A 450WXe lampwas

(27) Minella,M.; Faga,M.G.;Maurino, V.;Minero, C.; Pelizzetti, E.; Coluccia,S.; Martra, G. Langmuir 2010, 26, 2521–2527.(28) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1999, 15, 2402–

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1167–1172.(32) Li, D.; Huang, J. X.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135–145.(33) Kuwabata, S.; Takahashi, N.; Hirao, S.; Yoneyama,H.Chem.Mater. 1993,

5, 437–441.(34) Yoneyama, H. Adv. Mater. 1993, 5, 394–396.(35) Zhang, H.; Zong, R. L.; Zhao, J. C.; Zhu, Y. F. Environ. Sci. Technol. 2008,

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used for excitation, with the photon wavelength fixed at 250 nm.The PL spectra were collected in the 250-650 nm range using aphotomultiplier tube detector. Measurements were conducted inquartz cuvettes of 1 cm path length. The excitation and emissionslit widths were set at 3.0 mm.

Results and Discussion

Titanium oxide nanocrystals were synthesized by a modifiedsynthesis route involving the reaction of titanium chloride withpoly(ether glycol) of high molecular weight (i.e., with poly(1,4-butanediol)). This synthesis is based on the formation of partiallyhydrolyzed titanium alkoxide and the elimination of organichalite and HCl via a subsequent polycondensation reaction, asillustrated in Supporting Information Scheme S1.39,40 This reac-tion, which is performed at 120 �C, causes a stable colloidaldispersion of the TiO2 nanocrystals in the polymer solvent.Apparently, there is no clear evidence of nanocrystal formationbecause the resulting products have transparent features in visiblelight. This transparence is caused by the high affinity of thenanocrystals for the melted polymer. The addition of ethanol,tetrahydrofuran, or DMF did not change the degree of stability.However, the TiO2 nanocrystals in a melted polymer are easilydestabilized by the addition of acetone or chloroform. Theresulting turbid colloidal suspension is isolated by centrifugationin high yield and is washed several time with acetone.

Synthesized TiO2 was characterized by high-resolution trans-mission electron microscopy and powder X-ray diffraction. Thelow-magnification HRTEM image in Figure 1a shows the for-mation of nanocrystals with a truncated tetragonal bipyramidalWulff shape, and the highlymagnifiedHRTEMimage inFigure 1bindicates the formation of well-crystallized material. An anatasenanocrystal with theWulff shape contains {101} and {001} facets.The XRD analysis (Figure 1c) confirmed the presence of theanatase TiO2 phase only and a mean particle size of 8 nm, asestimated by the Scherrer formula. These nanocrystals form avery stable colloid in DMF with thixotropic behavior, as seen inFigure 1d. Apparently, the chemically attached polymer layer wasable to transfer its solubility to the nanocrystal. The nanocrystalcolloidal dispersion was used to perform DLS analysis, and theresult shows a narrow particle size distribution with an averagediameter of 9 nm (Supporting Information Figure 1S). Thissynthesis strategy makes promising TiO2 nanocrystals with goodphase and morphological control. However, we do not haveinformation about the chemical surface characteristics. To obtainthis information, a molecular probe test was performed. Polyani-line is an ideal molecular probe for identifying Brønsted acid sitesbecause of the presence of amine groups and an optical effect inthe presence of protons.

The molecular probe test proposed here is based on the simplereaction of the emeraldine base and TiO2 in an organic solvent.The main idea is to promote the reaction of the amine group ofpolyaniline with the nanocrystal by means of a protonationreaction, as illustrated in Scheme 1. For this purpose, TiO2 musthave a superficial Brønsted acid site, namely, a hydroxyl group,and it must be soluble in a suitable organic solvent in which theemeraldine base is also soluble, such as DMF. To obtain thehybridmaterial, the TiO2 solution wasmixed with the emeraldinebase solution at room temperature (Experimental Section). Mix-ing caused the solution to turn green, as illustrated in Figure 2c(vial 3c), indicating the protonation reaction and the formation of

emeraldine salt. The resulting hybrid material kept its colloidalstability and can be easily centrifuged (Supporting InformationFigure S2). It should be noted that the synthesis of TiO2

nanocrystals and the processing of the hybridmaterial are carriedout by simple, rapid routes using inexpensive raw materials.

FT-IR spectra of the hybridmaterial in Figure 3 confirmed thepresence of poly(1,4-butanediol) and emeraldine salt on thesurface of TiO2. The spectrum of TiO2 nanocrystals shows abroad band at 3260 cm-1, which is evidence of the terminalhydroxyl group of the polymer. The bands at 2967 and 2871 cm-1

are ascribed to the asymmetric and symmetric stretchingmodes ofthe C-H group. Two other important bands at 1649 cm-1

associated with the hydroxyl group of titanium oxide and at1104 cm-1 associated with the -C-O- group,14,41,42 relative topoly(1,4-butanediol), are observed.

The spectrumof thehybridmaterial (Figure 3) shows an intensesignal at 1656 cm-1 attributed to protonated dNþ (emeraldinesalt) absorbed on the nanocrystals’ surfaces and signals at 1572,1492, 1377, 1305, and 1251 cm-1 ascribed to polyaniline. This

Figure 1. (a) Low-magnification HRTEM image of the synthe-sized TiO2 nanocrystals. (b) High-magnification HRTEM imageshowing a well-crystallized material and the (101) plane of theanatase phase. (c) X-ray powder diffraction pattern of 8 nm TiO2

nanocrystals. (d) Colloidal TiO2 nanocrystals concentrated inDMF solvent after a washing process.

(39) Arnal, P.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A. Chem.Mater. 1997, 9, 694–698.(40) Shiihara, I.; Schwartz, W. T.; Post, H. W. Chem. Rev. 1961, 61, 1–30.

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analysis indicated that polyaniline and poly(1,4-butanediol) wereattached to the metal oxide surface. It is important to point outthat the infrared signals corresponding to the nitrogen bondshowed a shift in relation to the infrared spectrum of the pureemeraldine salt.43 This shift must be associated with the stronginteraction between polyaniline and the TiO2 surface.

A thermogravimetric analysis of the hybrid material showedthe presence of 12 wt % poly(1,4-butanediol) and 8 wt %

polyaniline. The formation of emeraldine salt is a clear indicationof the reaction of TiO2 with the emeraldine base. Because thereaction tookplace in anaprotic organic solvent such asDMF,webelieve that the presence of the OH supercial groups of thenanocrystal was the source of protons, acting as Brønsted acid

Figure 2. (a) UV-vis spectra of the TiO2 solution (black curve), emeraldine base (blue curve), and TiO2-emeraldine salt (green curve).(b)UV-vis spectra of the TiO2-emeraldine salt after being irradiated by visible light (purple curve) andUV light (red curve). (c) Photographof vials containing solutions of (1c) the emeraldine base, (2c) TiO2 nanocrystals, (3c) theTiO2-emeraldine salt, (4c) theTiO2-emeraldine saltafter UV-light irradiation, and (5c) the TiO2-emeraldine salt after visible-light irradiation. All solutions were prepared in DMF.

Scheme 1. Simplified Scheme of the Interaction of TiO2 Nanocrystals with Emeraldine Base to Form a Hybrid Material

(43) Ping, Z.; Nauer, G. E.; Neugebauer, H.; Theiner, J.; Neckel, A. J. Chem.Soc., Faraday Trans. 1997, 93, 121–129.

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sites. For a better analysis of the reaction between the emeraldinebase and TiO2, the material was characterized by UV-visspectroscopy, as indicated by the results in Figure 2a. This Figuredepicts the spectrum of the emeraldine base (blue curve), which isa typical spectrumwith two absorption peaks (325 and 625 nm),44

and the spectrum of TiO2 (black curve) showing an absorptionedge of around 340 nm associated with the electronic transitionbetween the valence band (VB) and the conduction band (CB) ofthis metal oxide. Now, looking at the spectrum of the hybridcompound TiO2-emeraldine salt (green curve), a shoulder ataround 430 nm and a broad peak beginning at 520 nm are visible.These two features are the fingerprint of the emeraldine salt,45

confirming the formation of the electronically conducting formofpolyaniline. In addition, note the red shift of the absorption edgeof the TiO2 nanocrystals, suggesting the formation of an interfacewith localized states. These localized states may be related to thetrapped charge at the TiO2-polyaniline interface.

The UV-vis and FT-IR spectroscopy analyses strongly sug-gest a protonation reaction between polyaniline and the TiO2

surface. However, for a more detailed analysis of this interaction,the material was characterized by X-ray photoelectron spectro-scopy, as shown inFigure 4. TheXPSanalysis shows the core-levelspectrum of Ti 2p3/2 of the hybrid compound, the TiO2-emeral-dine salt. This spectrum can be deconvoluted into two differentcurves, which can be ascribed to Ti4þ bound to oxygen, with apeak at 458.8 eV, and a Ti ion with a higher negative chargedensity, with a peak at 457.7 eV. A plausible explanation for thislower-energy peak of Ti is the deprotonation of Ti-OH, resultingin Ti-O-.46 It is important to point out that the core-levelspectrum of Ti 2p3/2 of the pure TiO2 nanocrystals showed thepresence of a single peak centered at 458.8 eV (Figure S3 in theSupporting Information). The core-level spectrum of N 1s alsoshowed relevant information about the protonation reaction, asillustrated in Figure 4b. TheN 1s spectrum of polyaniline showeda single peak centered at 398 eV, typical of a benzenoid amine,nondoped polyaniline, whereas the TiO2-emeraldine salt sampleshowed a spectrum with a peak centered at 401.5 eV associated

with positively charged nitrogen atoms of doped polyaniline.47-49

The XPS analysis corroborated the UV-vis and FT-IR results,strongly supporting the protonation reaction between the super-ficial OH group of TiO2 and the emeraldine base, resulting in theemeraldine salt.

The aforementioned results suggest that the hybrid materialpossesses the characteristics of the doped emeraldine salt, thus thehybrid material should present good electronic conductivity.Four-probe pressed-pellet conductivity for the TiO2-emeraldinesalt was 0.6 S cm-1, which is lower than the pure conventionalHCl-doped emeraldine powder with conductivity ranging from 1to 10 S cm-1.30 However, lower conductivity than that of pureemeraldine salt was expected because the total amount of polyani-line was only 8 wt%. Considering that the electronic conductivityof the hybrid material followed a phase role relationship and thatthe electronic conductivity of polyaniline is much higher than theconductivity of TiO2 anatase that ranges from 10-4 to 10-12 Scm-1, the conductivity of the hybrid material should range from0.2 to 2.0 S cm-1. On the basis of this analysis, the conductivitymeasurement corroborated the presence of doped material.

Figure 5 shows the photoluminescence (PL) spectra of the TiO2

nanocrystal in DMF and the TiO2-emeraldine salt hybridmaterial. This kind of characterization can supply informationabout the correlation of energy levels between polyaniline andTiO2. The PL spectrum of the TiO2 nanocrystals showed a broadasymmetric band with a maximum intensity at 380 nm (3.3 eV).This band can be associated with the emission process related tofree exciton electron-hole recombination (i.e., the maximumintensity must be associated with the band gap energy) ofTiO2.

49 The PL spectrum of the hybrid material showed twobroad asymmetric bands, one of lower intensity with a maximumat 413 nm (3.0 eV) and the other with a maximum at 493 nm(2.5 eV). The pure polyaniline did not showany PL in theUV-visrange of the spectrum. A plausible explanation for this PLemission behavior is as follows. The first band is presumablyassociated with emission from the localized states at theTiO2-polyaniline interface to the VB of TiO2. The emission bandat 413 nm supported the formation of localized states at theinterface, corroborating the UV-vis results in Figure 2a. Thesecondbandcanbe ascribed toan emissionprocess fromelectronsin theπorbital of the emeraldine salt to theVBof theTiO2.On thebasis of the PL measurement and on the optical transition of theemeraldine salt, we can propose that an electronically activeinterface was generated between polyaniline and TiO2, as illu-strated schematically in Figure 5b, considering electronic transi-tions, energy levels, and electron trapping. The relative positionsof the electronic energy levels proposed here make this hybridmaterial a good candidate for use in photovoltaic devices.50

The colloidal suspension of the hybrid material showed inter-esting optical activity when irradiated with UV or visible light. Asindicated in Figure 2b, when the green solution was irradiatedwith UV light it turned blue and when it was irradiated withvisible light it became purple. This photochromic behavior islikely related to light-induced oxidation. The UV-vis spectra ofthese solutions (Figure 2b) confirm the color change. Note, also,the red shift of the absorption edge in both samples, suggesting astrong interaction between polyaniline and TiO2 nanocrystals

Figure 3. Infrared spectra of (a) the emeraldine base, (b) the TiO2

nanocrystals synthesized in poly(1,4-butanediol), and (c) the hy-brid material (TiO2-emeraldine salt).

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even after oxidation took place. It is also important to note thatthe hybrid material retained its colloidal stability after its colorchanged.

Conclusions

The reaction between poly(1,4-butanediol) and TiCl4 led to theformation of colloidal nanocrystals with a polymer layer and acidsites on the surface during the synthesis process. The polymerchemically attached to the nanocrystals’ surfaces and Brønstedacid sites allowed high stability in DMF and the capacity to reactwith an amine compound, such as polyaniline. A colloidaldispersion of the hybrid material showed interesting opticalactivity when irradiated with UV or visible light. As indicatedin Figure 2, when the green solution was irradiated with UV lightit turned blue and when it was irradiated with visible light itbecame purple. This photochromic behavior is likely related to

light-induced oxidation. The UV-vis spectra of these solutionsconfirm the color change.Note also the red shift of the absorptionedge in both samples, suggesting a strong interaction betweenpolyaniline and TiO2 even after oxidation took place. It is alsoimportant to note that the hybrid material retained its colloidalstability after its color changed.

Acknowledgment.The financial support of FAPESP (projects98/14324-0 and 07/51002-2), FINEP, CNPq, and CAPES (allBrazilian agencies) is gratefully acknowledged.

Supporting Information Available: Reaction mechanism,DLS analysis, photograph of centrifuged hybrid material,and Figures of XPS spectra of TiO2 nanocrystals. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Figure 4. (a)Core-level spectrumofTi 2p3/2 of the hybridmaterial (TiO2-emeraldine salt). (b)Core-level spectrumofN1sof the polyaniline(red curve) and hybrid material (TiO2-emeraldine salt, black curve).

Figure 5. (a) PL spectra for the colloidal TiO2 dispersion and hybrid material (TiO2-emeraldine salt), both in DMF (250 nm excitationwavelength). (b) Schematic representation of the generation of an electronically active interface considering the electronic levels of theTiO2-emeraldine salt (1) emission process related to the free exciton electron-hole recombination (CB to VB transition), (2) emission fromthe localized states at the TiO2-polyaniline interface to the VB of TiO2, (3) emission from the trapped electrons in the π orbital of theemeraldine salt to the VB of the TiO2.